The actors wear chunky, patterned jumpers, it is filmed largely in the dark and rain and it has a less than inviting title.

But BBC4's The Killing, a subtitled Danish thriller that slowly unfolds over 20 hours as police hunt for the murderer of a 19-year-old girl, has proved a perhaps unlikely hit. The show has been getting higher viewing figures than Mad Men did when it was shown on the channel. The BBC has confirmed that it has bought the second series and will show it later this year.

"It is a diamond of a series – complex, dramatic, thoroughly gripping," said BBC4 controller Richard Klein.

The show, which has been a hit across Europe, underscores the growing popularity of the Scandinavian crime genre, led by the late author Stieg Larsson and the Swedish drama Wallander.

"I'm not one of those who believe that it will remain this popular but I do think it will still grow for a few years yet," said Jakob Stougaard-Nielsen, a lecturer in Scandinavian literature at UCL. "There's some really undiscovered crime writers who already sell well abroad but haven't reached the UK yet, such as the Danish Jussi Adler-Olsen."

As ratings and praise for The Killing grow, Sarah Lund, the slightly scruffy and decidedly uncommunicative detective at the centre of the Emmy-nominated drama, has quickly become a cult figure.

Sofie Gråbøl, who plays Lund in the show – called Forbrydelsen in Danish – explained its appeal: "When this genre is done well you can keep the audience interested by the whole guessing thing – but at the same time talk about some very dark aspects of the human psyche."

But part of the appeal of the series is a focus on aspects not usually examined by English-language equivalents.

"With The Killing it's almost all about how colleagues are interacting, and the families – both of the policeman and the family that has been exposed to violence – and in crime drama that's something you don't usually see," said Stougaard-Nielsen.

The programme – which mixes politics, policework and a focus on a family left devastated by violent crime – has been out-rating both BBC4's Swedish Wallander imports and the much-lauded Mad Men, which the channel lost to Sky Atlantic late last year.

In part that is due to Gråbøl's portrayal of Lund, a female TV detective in a canon dominated by men.

"The first images I got in my head were based on cliches about female detectives … We have images of a woman in a suit, or a woman in a man's world," said Gråbøl, who decided to play Lund as neither.

Instead Lund comes clothed in a distinctly Scandinavian handknitted jumper, fast becoming a must-have item for fans of the show. "Everybody wanted that sweater [in Denmark]. The company in the Faroe Islands couldn't keep up," said Gråbøl.

As well as benefitting Scandinavian authors, the crime boom has created an opportunity for UK producers. Kenneth Brannagh's portrayal of Henning Mankell's crumpled Inspector Kurt Wallander for BBC1 has been acclaimed critically, attracted audiences of around 6 million, and been sold to countries around the world – including Sweden.

"I'm told they have the same debate that we have here," said Francis Hopkinson, the show's producer at Left Bank Pictures. "Some people much prefer the English Wallander and the some the Swedish."

The company is working on a further six feature-length Wallander mysteries, including an adaptation of the forthcoming – and final – novel, and a new story based on an original premise from Mankell.

The Killing, too, is soon to air as an English-language production. The American broadcaster AMC, which also makes Mad Men, has remade the show and it will begin in the US this spring, although it is unlikely to be shown on the BBC.

"I don't think we will acquire it," said Sue Deeks, the BBC's head of acquisitions. "It seems very close to the original version. I don't think we'd get the same response twice."

It's unclear whether we are likely to see more programming that attempts to capitalise on Scandinavian crime's current popularity, second series of The Killing aside.

For now, all eyes are glued on detective Sarah Lund. Speculation about The Killing's finale grew so intense in Denmark that fans placed large bets on the killer.

"I had a big moral dilemma – should I play and make millions?" Gråbøl jokes. "I didn't. But people did bet a lot of money."

Small screen subtitles

While foreign language film finds a distribution in Britain – albeit limited – the same has not always been the case for TV. The way for the current wave of programming on BBC4 was paved by Spiral, often described as France's answer to The Wire, which the channel first began screening in 2006.

"When we first acquired Spiral (about to enter its third series), the success of that made us look at other Euro crime drama and led us on to Wallander – and that's led us to The Killing," said Sue Deek, the corporation's head of acquisitions.

Ratings for foreign language imports on BBC4 have been growing year on year, with The Killing averaging at 500,000. "I just think viewers like to see it in the original language." Does that mean viewers are warming to the idea of subtitles? "Maybe what's happening is that more people who might not have considered watching something with subtitles have been drawn in and find that it's perfectly OK." The corporation has shown subtitled programming sporadically. Between 1986 and 1993, BBC2 screened the cult German series Heimat, which told the turbulent history of the country in the 20th century through the lives of a single family. The same channel screened Lars Von Trier's Kingdom in 1997 and in the early 1990s showed Krzytof Kieslowski's Dekalog (Ten Commandments).

Perhaps the most memorable was Das Boot, the German drama set on a submarine, which first screened on BBC2 in 1984. Jakob Stougaard-Nielsen, a lecturer in Scandinavian literature, said he had been surprised at The Killing's success and that British viewers had the patience to wait for a long-running plot to unfold and read subtitles. It depends, he added, on the quality of the translation – and the original writing. "If the dialogue is good then you don't mind reading it on screen. If the dialogue is rubbish you do."

The intricate plot and gritty naturalism of The Killing have had BBC4 and iPlayer viewers enthralled for weeks – and at its heart is detective Sarah Lund, played by Sofie Gråbøl. 'How did I come up with the character?' she asks. 'By acting like a man'